FIG. 1 is a cross-sectional view of a conventional OLED. Referring to FIG. 1, a conventional OLED 100 generally includes an anode 110, an organic layer 120, and a cathode 130. The anode 110 is disposed on a substrate 10. The substrate 10 may have at least one thin film transistor (not shown) connected to the anode 110. The organic layer 120 is disposed on the anode 110 and includes an organic emission layer 123. The organic layer 120 may have several component layers, but generally includes a hole injection layer 121, a hole transport layer 122, the organic emission layer 123, an electron transport layer 124, and an electron injection layer 125. The cathode 130 is disposed on the organic layer 120.
When a voltage is applied between the anode 110 and the cathode 130, a hole is injected from the anode 110 into the hole injection layer 121. The injected hole is transported into the organic emission layer 123 via the hole transport layer 122.
Also, an electron is injected from the cathode 130 into the electron injection layer 125 and then transported into the organic emission layer 123 via the electron transport layer 124.
The hole and the electron combine in the organic emission layer 123 to generate excitons which then transition from their excited state to the ground state to emit light.
The hole transport layer 122 should facilitate hole injection and have good hole transport properties. Also, the hole transport layer 122 may block the transfer of electrons and has a high glass transition temperature (Tg) to maintain stability even when the temperature varies.
To lengthen the OLED's life span (which is the device's biggest disadvantage), it is necessary to prevent diffusion of oxygen, ions, or small molecules from the hole transport layer 122. Accordingly, a dense thin film having a high glass transition temperature (Tg) may be formed, thereby minimizing the deterioration of the life span due to the hole transport layer. Examples of such methods for lengthening the life span of the OLED by improving the hole transport layer 122 include doping a fluorescent illuminant on the hole transport layer 122, and doping a fluorescent illuminant on the hole transport layer 122 and the organic emission layer 123.